Draft: Deduplication pipeline creation for external pull requests

What does this MR do and why?

This attempts to address #352158 by deduplicating CI pipeline creation when an external pull request calls the pipeline creation worker

We prevent creating further pipelines if 50 or more running pipelines already exist for the same SHA/ref.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #352158